Add a setting to do frame scrolls through the root layer.

Source/core/rendering/RenderLayerClipper.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 26 matching lines @@
  * version of this file under the LGPL, indicate your decision by
  * deletingthe provisions above and replace them with the notice and
  * other provisions required by the MPL or the GPL, as the case may be.
  * If you do not delete the provisions above, a recipient may use your
  * version of this file under any of the LGPL, the MPL or the GPL.
  */
 
 #include "config.h"
 #include "core/rendering/RenderLayerClipper.h"
 
+#include "core/frame/Settings.h"
 #include "core/rendering/RenderLayer.h"
 #include "core/rendering/RenderView.h"
 
 namespace blink {
 
 static void adjustClipRectsForChildren(const RenderObject& renderer, ClipRects& clipRects)
 {
     EPosition position = renderer.style()->position();
     // A fixed object is essentially the root of its containing block hierarchy, so when
     // we encounter such an object, we reset our clip rects to the fixedClipRect.
@@ skipping 208 matching lines @@
         // Clip applies to *us* as well, so go ahead and update the damageRect.
         LayoutRect newPosClip = toRenderBox(m_renderer).clipRect(offset);
         backgroundRect.intersect(newPosClip);
         foregroundRect.intersect(newPosClip);
         outlineRect.intersect(newPosClip);
     }
 }
 
 void RenderLayerClipper::calculateClipRects(const ClipRectsContext& context, ClipRects& clipRects) const
 {
-    if (!m_renderer.layer()->parent()) {
+    bool rootLayerScrolls = m_renderer.document().settings() && m_renderer.document().settings()->rootLayerScrolls();
+    if (!m_renderer.layer()->parent() && !rootLayerScrolls) {
         // The root layer's clip rect is always infinite.
         clipRects.reset(PaintInfo::infiniteRect());
         return;
     }
 
     bool isClippingRoot = m_renderer.layer() == context.rootLayer;
 
     // For transformed layers, the root layer was shifted to be us, so there is no need to
     // examine the parent. We want to cache clip rects with us as the root.
     RenderLayer* parentLayer = !isClippingRoot ? m_renderer.layer()->parent() : 0;
@@ skipping 78 matching lines @@
         ASSERT(current);
         if (current->transform() || current->isPaintInvalidationContainer())
             return const_cast<RenderLayer*>(current);
     }
 
     ASSERT_NOT_REACHED();
     return 0;
 }
 
 } // namespace blink